1. Field of the Invention
The present invention relates to a process of producing glutamate derivatives using enzyme reactions. Further, the invention relates to a process of producing monatin using tryptophan as a starting material.
2. Discussion of the Background
4-(Indol-3-ylmethyl)-4-hydroxy-glutamic acid (3-(1-amino-1,3-dicarboxy-3-hydroxy-butan-4-yl)-indole) (hereinafter referred to as “monatin”) represented by the following structural formula (6) is an amino acid contained in the root of a shrub tree (e.g., Schlerochitom ilicifolius) in South Africa. Monatin has a level of sweetnessthat is several hundreds-fold greater than that of sucrose. Therefore, monatin is a particularly promising low-calorie sweetener (see JP-A-64-25757).

Monatin has asymmetric carbons at positions 2 and 4. Naturally occurring monatin has a (2S, 4S) steric configuration. In addition to the (2S, 4S) steric configuration, the non-naturally occurring stereoisomers may be synthetically produced. It has been determined that all of the other stereoisomers of monatin have a highly enriched sweetness level compared to sucrose. Therefore, it is expected that any single monatin stereoisomer or mixture of monatin stereoisomers may be utilized as a sweetening agent or as a component for a sweetening agent (e.g., a sweetener).
Five examples of monatin production processes have been reported. The details of these processes are as described in the following references of the related art.                (1) Specification of U.S. Pat. No. 5,994,559        (2) Tetrahedron Letters, 2001, Vol. 42, No. 39, pp. 6793-6796        (3) Organic Letters, 2000, Vol. 2, No. 19, pp. 2967-2970        (4) Synthetic Communication, 1994, Vol. 24, No. 22, pp. 3197-3211        (5) Synthetic Communication, 1993, Vol. 23, No. 18, pp. 2511-2526        
However, each of these processes requires multiple steps. Moreover, an industrial-scale production process is yet to be established. Thus, a demand exists for the development of a simple, high-yield industrial process for producing glutamate derivatives, including monatin and analogs thereof.
Thus, it is an object of the invention to provide an efficient process of producing glutamate derivatives and salt forms thereof including monatin. These compounds are promising as a component for a sweetener.